Ultra fine particles having uneven surfaces and treatment plate using same

ABSTRACT

An ultrafine particle film which is effective for the prevention of an electrostatic charge and reflection and can be applied to a large area at a low cost, a cathode ray tube to which the ultrafine particle film is applied, and image display plate to which the ultrafine particles is applied, and a process for producing the ultrafine particle film which comprises attaching a coating solution bath to a Braun tube surface and the like, introducing a containing solution containing anti-reflection SiO 2  ultrafine particles and antistatic SnO 2  ultrafine particles, and elevating up or down on the Braun tube surface and the like at a constant rate.

This application is a Continuation of application Ser. No. 07/855,055,filed Mar. 19, 1992 (now abandoned) which is a continuation- in-part ofSer. No. 07/404,553, filed Sep. 8, 1998 (now U.S. Pat. No. 5,189,337).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transparent plate, a process for theproduction thereof and a transparent plate-applied screen display plate,and a Braun tube (cathode ray tube) and a process for the productionthereof. In particular, it relates to a transparent plate usingultrafine particles capable of effectively functioning as an antistaticand anti-reflection film for a screen display plate, a process forproducing the same and a transparent plate-applied screen display plate,and a Braun tube (cathode ray tube) and a process for the productionthereof.

2. Related Art

Films to reduce the reflectance of a transparent plate surface have beenlong studied, and have been applied to lenses for cameras and ophthalmicglasses. At present, such films are used as an anti-reflection filterfor reducing the reflected light on VDTs (visual display terminal). Avariety of anti-reflection films have been considered, and mainly usednow are multi-layered films and heterogeneous films.

A multi-layered film has a structure in which a material having a lowreflectance and a material having a high reflectance are alternatelystacked to form at least three layers. Its anti-reflection effect is asynergistic effect produced by the optical interference function of eachlayer. Multi-layered films are discussed in Physics of Thin Films, 2(1964), pp. 243-284.

A heterogeneous film having a reflectance distribution in the filmthickness direction works as an anti-reflection when the film has alower refractive index than a glass base plate. A heterogeneous film isgenerally formed by rendering a transparent plate surface porous.

Apl. Phys. Lett., 36 (1980), pp. 727-730 discusses a method of reducingthe reflectance in which a heterogeneous film is produced by forming aninsular metal deposition film on a glass surface and forming a fineuneven surface by sputter etching.

Solar Energy 6 (1980), pp. 28-34 discusses a method of reducing thereflectance in which a soda glass surface is rendered porous by dippingit in an H₂ SiF₆ solution oversaturated with SiO₂.

On the other hand, in a cathode ray tube, it is required not only toform an electrically conductive film for prevention of electrostaticcharge but also to use devices for prevention of reflection.

Meanwhile, it is known that the front panel surface (image displayplate) of a cathode ray tube such as a Braun tube is electrostaticallycharged. The reason therefor is as follows. Aluminum is generallydeposited to form a thin and uniform film on a phosphor 43 applied to aninner surface 42 of a Braun tube 41 as shown in FIG. 4. In theapplication of a high voltage to the aluminum film 44, an electrostaticcharge occurs on a front panel 45 of the Braun tube due to electrostaticinduction when the high voltage is applied and cut off.

JP-A-61-51101 discloses a method of forming an antistatic andanti-reflection film for prevention of both electrostatic charge andreflection on such a display tube surface. In this method, first, anelectrically conductive film is formed on a glass base plate by aphysical gas phase method or a chemical gas phase method such as avacuum deposition method and a sputtering method, and then, ananti-reflection film is formed thereon.

In the above prior arts, the film forming method is limited to asputtering or vacuum deposition method, and it is required to controlthe film thickness highly accurately. There is therefore a defect inthat a high cost is required and it is difficult to apply these priorart methods to a base plate having a large surface area.

In particular, the above prior art method uses a two-layer structure, inwhich an electrically conductive film and an anti-reflection film areformed, respectively. There have been therefore problems in productivityand cost. Further, when such films are formed on the surface of adisplay tube such as a Braun tube which limits the firing temperaturefor forming films to low temperatures, there have been problems in filmstrength and reflectivity.

In a reflection film containing ultrafine particles, a minimumreflectance is obtained when the ultrafine particles are highly denselyand regularly arranged on a base plate.

FIG. 5 schematically shows a cross-sectional view of a film in whichultrafine particles are systematically and regularly applied to atransparent base plate. In FIG. 5, numeral 46 indicates ultrafineparticles, numeral 47 indicates a binder layer, and numeral 48 indicatesa base plate. In this Figure, n₀ is the refractive index of air, n₁ isthe refractive index of an ultrafine particle layer, da, on the airside, n₂ is the refractive index of the da layer on the ultrafineparticle side, n_(S) is the refractive index of a layer formed fromultrafine particles and a binder, and n_(G) is the refractive index ofthe transparent base plate. In this case, the reflectance, Ra, of the dalayer is represented by the equation (expression 1), and the reflectiveindex, Rb, of the db layer, by the equation (expression 2). ##EQU1##

When the reflectance of a portion where no ultrafine particles arepresent is taken as Rc, the total reflectance is represented by theequation (expression 3),

    R=(1-α)(Ra+Rb)+Rc                                    (Expression 3)

in which α is the ratio of an area where no ultrafine particles arepresent.

When the binder is a vitreous binder, Rc is generally 4.2%.

Ra is about 0.19% at λ=550 nm on the assumption that n₀ =1.0, n₁ =1.10,n₂ =1.38 and n_(S) =1.47. When the transparent plate is glass, Rb isabout 0.04% at λ=550 nm on the assumption that n_(G) =1.53 and that theother refractive indexes are the same as those in Ra.

The consequence is (Ra+Rb)<Rc. That is, it is understood that thesmaller α is, the smaller the reflectance is. In other words, whenultrafine particles are regularly and densely applied, the reflectanceis the lowest.

The present inventors have already proposed the application of ultrafineparticles to an anti-reflection film and filed on Sep. 8, 1989 as U.S.Ser. No. 07/404553 (U.S. Pat. No. 5,189,337) whose content isincorporated herein by reference. As a result of a further study, it hasbeen also found that a coating solution level is elevated up or down onthe base plate surface at a constant rate, whereby ultrafine particlescontained in the coating solution are regularly arranged on, and appliedto, the base plate surface to give a low reflectance close to atheoretical value.

It has been also found that, in the above case, ultrafine particleshaving an uneven surface are used, whereby there is obtained a filmwhich shows a decrease in diffuse reflection and is not opacified.

It has been further found that, in the above case, antistatic ultrafineparticles of which the diameter is not more than 1/10 of that of theanti-reflection ultrafine particles are incorporated, whereby antistaticultrafine particles are arranged in a network form in gaps among theanti-reflection ultrafine particles to form an electrically conductivefilm.

The present invention provides an antistatic and anti-reflection filmwhich can be applied to a large area at a low cost and an image displayto which the film is applied.

The present invention is achieved either by filling a coating solutioncontaining uniformly dispersed ultrafine particles in a bath positionedon the side of a surface of a base plate and pulling up the base plateat a constant rate, or by filling the above coating solution in a bathpositioned on the side of a surface of a base plate at a constant rate.

The present invention is achieved by making uneven surfaces ofanti-reflection ultrafine particles, or by at least making the surfacesporous. Otherwise, uneven portions may be formed by aggregatingultrafine particles to form fine particles which have gaps formed of theultrafine particles on surfaces of the fine particles.

The process for producing a transparent plate, provided by the presentinvention, has a characteristic feature in that a film composed ofultrafine particles and a binder to be filled in gaps among saidparticles is formed on a transparent base plate by resting the baseplate against a container, introducing a mixed coating solutioncontaining said particles and said binder into the container to elevatethe mixed coating solution level up on the base plate surface at aconstant rate, or lowering the mixed coating solution level on the baseplate surface at a constant rate, to form the ultrafine particle film onthe base plate surface.

The transparent plate of the present invention has the followingcharacteristic features. In a transparent plate obtained by forming anultrafine particle film of ultrafine particles and a binder filled ingaps among the ultrafine particles on a transparent base plate, acoupling agent having a functional group to a material of the base plateis incorporated into the ultrafine particle film and/or allowed topresent in the interface between the ultrafine particle film and thetransparent base plate, and/or a coating liquid composed mainly of ethylsilicate is further applied onto a coating of the ultrafine particles.

The image display plate and the image display protection plate providedby the present invention have a characteristic feature in that theabove-prepared transparent plate is applied to the surface of thetransparent base plate or that the ultrafine particle film is directlyformed on the image display plate surface.

The Braun tube of the present invention has a characteristic feature inthat it is formed from the above image display plate or the above imagedisplay protection plate. In addition, the present invention can be alsoapplied to other cathode ray tubes, liquid crystal display devices,window glass for automobiles and ophthalmic glasses, and the like.

The process for producing the Braun tube, provided by the presentinvention, has a characteristic feature in that the transparent baseplate surface of the Braun tube is exposed through an opening portionprovided on the side of a container, and a mixed solution containingultrafine particles and a binder is introduced into the container toelevate the mixed coating solution level up on the base plate surface ata constant rate, or lowering the mixed coating solution level on thebase plate surface at a constant rate, to form an ultrafine particlefilm on the base plate surface.

The process for producing the window glass for an automobile has acharacteristic feature in that the transparent base plate surface of thewindow glass for an automobile is exposed through an opening portionprovided on the side of a container, and a mixed solution containingultrafine particles and a binder is introduced into the container toelevate the mixed coating solution level up on the base plate surface ata constant rate, or lowering the mixed coating solution level on thebase plate surface at a constant rate, to form an ultrafine particlefilm on the base plate surface.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a layout drawing of a device of one embodiment of the presentinvention.

FIG. 2 shows reflectance characteristics of an anti-reflection film towhich an ultrafine particle film of the present invention has beenapplied.

FIG. 3 shows antistatic characteristics of an antistatic film to whichan ultrafine particle film of the present invention has been applied.

FIG. 4 is a cross-sectional view of a cathode ray tube as one embodimentto which the present invention has been applied.

FIG. 5 is a schematic cross-sectional view of one embodiment of theultrafine particle film of the present invention.

FIG. 6 shows film thickness characteristics of one embodiment of theultrafine particle film of the present invention.

FIG. 7 shows a layout drawing of another embodiment of the presentinvention.

FIG. 8 is a schematic cross-sectional view of one embodiment of thedevice of the present invention.

FIG. 9 is a cross-sectional view of one embodiment of theanti-reflection film of the present invention.

FIG. 10 shows characteristics of a change in refractive index in thefilm thickness direction of an anti-reflection film.

FIG. 11 shows characteristics of reflectance of one embodiment of theanti-reflection film of the present invention and reflectance of anuntreated glass plate at a wavelength of 400 to 700 nm.

FIG. 12 is a cross-sectional view of one embodiment of the presentinvention.

FIG. 13 shows a particle size distribution of one embodiment having aparticle size distribution.

Numerals in the above Figures are as follows. 11 . . . Braun tube, 12 .. . coating bath, 13 . . . coating solution, 14 . . . pressure-adjustingvalve, 15 . . . overflowing valve, 16 . . . solution tank, 17 . . .solution-feeding pressure valve, 18 . . . leaking valve.

DETAILED DESCRIPTION OF THE INVENTION

It is generally known that when a film is formed from a coating solutioncontaining no ultrafine particles by a dipping method, the followingequation (Expression 4) is established between the thickness, t, and thepulling-up rate, v,

    t=K(ηv/pg).sup.0.5                                     (Expression 4)

wherein n is the viscosity of a solution, p is the density of thesolution, g is gravitational acceleration, and K is a constant.

On the other hand, the present inventors have found that when the levelof a mixed coating solution containing ultrafine particles is elevatedup or down on the base plate surface at a constant rate, there is formeda film having structure in which the ultrafine particles are arranged inone layer up to a certain pulling-up rate and that when the pulling-uprate increases more, there is formed a film having a structure in whichthe ultrafine particles are arranged in two or more layers, as is shownin FIG. 6.

Therefore, when ultrafine particles, D₂, are incorporated as ultrafineparticles for prevention of reflection, the resultant film has anapparent thickness of D₂ if the pulling-up rate is within the range inwhich one layer is formed. In this case, the critical rate is about 10mm/s.

It is very difficult to apply a coating solution to form a film on thesurface of a base plate having a complicated form such as a Braun tubeby a usual dipping method. However, the present invention uses a bathcontaining a coating liquid in which a Braun tube surface is placed, andthe coating liquid level is elevated up or down at a constant level,whereby a coating can be easily formed.

When a film is formed from a mixture of ultrafine particles, eachcomponent of the mixture exhibits its function. For example, when themixture contains a large amount of ultrafine particles having ananti-reflection function, the resultant film mainly exhibits alow-reflection function due to surface coarseness. Further, when theultrafine particles as other component of the mixture are electricallyconductive ultrafine particles having a size of not more than 1/10 ofthe anti-reflection ultrafine particles, the resultant film alsoexhibits electrical conductivity due to an effect of their aggregatingin a network form. Accordingly, the process of the present inventiongives a film having two characteristics of anti-reflection andantistatic functions when the mixed solution is applied only once.

Further, when the mixed coating solution contains ultrafine particles ofwhich the diameter is 2 to 3 times the diameter of the ultrafineparticles as a major component, the former ultrafine particles areinsularly arranged. This state is comparable to a state in which islandsof relatively large ultrafine particles are sparsely spotted in the seaof anti-reflection ultrafine particles.

This distance among the relatively large ultrafine particles is about 1to 2 μm. Therefore, when the film undergoes a mechanical friction force,these larger ultrafine particles are in contact, and the anti-reflectionultrafine particles are not in contact. As a result, no optical changeoccurs. Namely, the optical strength of the film against friction isremarkably improved. For the same reason, the easiness to remove dirtsuch as an oil is also improved.

When one touches a submicron-order uneven surface of a film with hands,a finger mark is transferred to the film, and it is difficult to removeit even with an alcohol. When, however, the above relatively largeultrafine particles are incorporated and insularly arranged, a fingermark is hardly transferred, and even if transferred, it can be easilycleaned off.

Further, the degree of diffuse reflection can be decreased by devisingan uneven surface, forming a porous surface and aggregating ultrafineparticles. The diffuse reflection refers to reflections in all thedirections. According to the present invention, when a light pathreaches a concave portion of the ultrafine particles, light no longerproceeds and terminates itself within the concave portion. In thediffuse reflection, the coated film looks opaque. At the same time, thetransmissivity is deteriorated, and the resolution is decreased.

It is desirable to reduce the diffuse reflectance to zero whileimproving the resolution, but it is actually difficult to do so. Whenthe ultrafine particles are replaced with further ultrafine particles(average particle diameter of 0.01 μm or less), the diffuse reflectanceis decreased to zero. However, the regular reflectance comes close tothat of the nontreatment state. Therefore, it is our proposal to form anuneven surface (including a porous surface and aggregation of ultrafineparticles) in order to decrease the diffuse reflectance toward zerowhile maintaining the size of the ultrafine particles.

The present invention will be described with regard to its embodimentsby reference to drawings. First, the constituent requirements of thepresent invention will be explained.

(Ultrafine particles)

The ultrafine particles are not specially limited in function as far asthe transparency and the translucency are not hindered, but refer tothose having an average particle diameter of not more than 1 μm. Typicalfunctions thereof are antistatic, anti-reflection and/orinfrared-reflection.

The material for the antistatic ultrafine particles is preferablyselected from the group consisting of SnO₂ (tin oxide), SnO₂ +Sb₂ O₃(antimony oxide), In₂ O₃ (indium oxide), and In₂ O₃ +SnO₂. The materialfor the anti-reflection ultrafine particles is preferably selected fromthe group consisting of SiO₂ (silicon dioxide) and MgF₂ (magnesiumfluoride). The material for the infrared-reflection ultrafine particlesis preferably selected from the group consisting of SnO₂, SnO₂ +Sb₂ O₃,In₂ O₃, In₂ O₃ +SnO₂, TiO₂ (titanium oxide) and ZrO₂ (zirconium oxide).

The above anti-reflection ultrafine particles preferably have an averageparticle diameter of 100 to 150 nm (0.1 to 0.15 μm). When this averageparticle diameter is less than 100 nm (0.1 μm), SiO₂, etc., may give afilm having too flat an outermost surface, and no sufficientanti-reflection effect may be obtained. When it is more than 150 nm(0.15 μm), a sufficient anti-reflection effect can be obtained, butdiffuse reflection increases. As a result, the film is opacified, and atthe same time, the resolution may be decreased. Therefore, the particlediameter of the anti-reflection ultrafine particles is preferably 100 to150 nm (0.1 to 0.15 μm). All the materials, SiO₂, MgF₂, etc., for theanti-reflection ultrafine particles have a refractive index of not morethan 1.50. In this case, when ultrafine particles having uneven surfacesare used, scattered light on the ultrafine particle surfaces decreases.As a result, the diffuse reflection of the film as a whole greatlydecreases, and the opacification can be overcome. The uneven surfaces ofthe ultrafine particles may be formed by any one of a method in whichthe surfaces of the ultrafine particles are made porous as shown in FIG.8(a), a method in which smaller ultrafine particles are added to arelatively larger ultrafine particle as shown in FIG. 8(b), a method inwhich an aggregate of at least three ultrafine particles is formed, andother methods.

The antistatic ultrafine particles preferably have an average particlediameter of not more than 10 nm (0.01 μm). The ultrafine particles ofdifferent kinds may be used in combination. The particle diameter of theantistatic ultrafine particles used in combination of theanti-reflection ultrafine particles is preferably not more than 1/10 ofthe particle diameter of the anti-reflection ultrafine particles. Thatis, when a mixed solution containing two kinds of ultrafine particleshaving different particle diameters is applied, two kinds of theultrafine particles of which the diameter ratio is within 1/10 aredispersed relatively well. However, when the diameter ratio is more than1/10, the ultrafine particles having a smaller diameter aggregate in anetwork form without being dispersed.

Therefore, when the diameter of the antistatic ultrafine particles iswithin 1/10 of that of the anti-reflection ultrafine particles, thesetwo kinds of ultrafine particles are dispersed to such an extent thatthe electrical conductivity is lost. As a result, the resultant filmexhibits no antistatic function. On the other hand, when the diameterthereof is not less than 1/10 in the ratio, the antistatic ultrafineparticles aggregate in a network form, and the resultant film iselectrically conductive and excellently exhibits an antistatic function.In the present invention, the anti-reflection ultrafine particles have adiameter of 100 to 150 nm (0.01 to 0.15 μm). Therefore, the diameter ofthe antistatic ultrafine particles is preferably not more than 19 nm(0.019 μm).

In the amount ratio of the anti-reflection ultrafine particles and theantistatic ultrafine particles, the amount of the antistatic ultrafineparticles is preferably not less than 10 percent of the total amount ofthe ultrafine particles. When this amount exceeds 50%, theanti-reflection function may be decreased, and it is therefore requiredto be not more than 50%.

For the same reason, it is preferred to incorporate not more than 20% byweight, based on the total amount of the ultrafine particles, ofultrafine particles of which the diameter is 2 to 3 times the diameterof the anti-reflection ultrafine particles. In addition, the "ultrafineparticles" used in the present invention may refer to compositeparticles composed from at least two inorganic oxides, and there may beused ultrafine particles which are a mixture of at least two inorganicoxides or have particle structure in which one particle is included inother particle, and have an average particle diameter of not more than0.1 μm. Preferred are ultrafine particles which satisfy the followingconditions; the particle size distribution has a maximum peak inparticles having particle diameters in the vicinity of their averageparticle diameter, the particles having an average particle diametermake up at least 50 percent of the particles in total, the maximumdiameter is nearly twice the average particle diameter, and the minimumparticle diameter is about 1/2 of the average particle diameter. Theaverage particle diameter of a finer component (corresponding to asolute) included in each ultrafine particle (corresponding to a solvent)is preferably 0.01 to 0.05 μm.

The above ultrafine particles may be not only spherical but alsodefectively spherical. However, when the ultrafine particles have toosmall a diameter, the resultant film has too smooth an outermostsurface, and no sufficient anti-reflection effect may be obtained. Theaverage particle diameter is preferably not less than 0.05 μm.Meanwhile, when the average particle diameter is too large, thediffusion effect is too high, the resolution is decreased, and the filmstrength is decreased. Therefore, the average diameter is preferably notmore than 0.1 μm. The typical combination of at least two kinds of theabove ultrafine particles is a combination of an electrically conductivecomponent with an anti-reflection component. The amount ratio of theelectrically conductive component and the anti-reflection componentdiffers to some extent depending upon manufacture conditions. However,the proportion of the electrically conductive component is preferably atleast 10% (at least 0.1 in a volume ratio) of the total weight of theultrafine particles. When the above proportion exceeds 50%, theanti-reflection function may be decreased, and therefore, the aboveproportion is required to be adjusted to not more than 50%. Theelectrically conductive component is sometimes referred to as "minorcomponent", and the anti-reflection component, as "major component"hereinafter.

It is not clear how these components to constitute the ultrafineparticles of the present invention form the particle product or whetherthese components necessarily form a constant form depending upon thekinds and performances of the components. The minor component in aparticle form is sometimes included in the major component, and in thiscase, the average particle diameter of the particle formed of the minorcomponent is 0.01 to 0.05 μm.

The combination of these components is not limited to theabove-described combination. The point is that the ultrafine particlessatisfy the two functional requirements. When the minor component isincluded in the major component as described above, such a state iscomparable to a state in which the ultrafine particle(s) composed of themajor component is (are) the sea and the ultrafine particles of theminor component are present like islands. Further, when not more than10% by weight of fine particles which have an average particle diameterof 0.01 to 0.05 μm and are composed of an electrically conductivecomponent or a combination of an electrically conductive component withan anti-reflection functional component are incorporated to theultrafine particles of the present invention, there is obtained the sameeffect as that obtained by using the ultrafine particles of the presentinvention alone.

The maximum peak of the particle size distribution is preferably in thevicinity of the average particle diameter, the particles having anaverage particle diameter make up at least 50 percent of the particlesin total, the maximum diameter is nearly twice the average particlediameter, and the minimum particle diameter is about 1/2 of the averageparticle diameter.

The particle diameter (average particle diameter) of the above ultrafineparticles (SiO₂ particles in particular) is restricted in terms of theimage resolution and the anti-reflection effect on external light. Thelower limit is determined in view of the anti-reflection effect. Whenthe particle diameter less than 50 nm (0.05 μm), it is difficult toobtain the intended anti-reflection effect. The upper limit isdetermined in view of the resolution. When the particle diameter exceeds1 μm, the resolution is greatly decreased. The above range has beentherefore determined as a range in practical use. The particle diameteris preferably 50 nm to 300 nm (0.05 μm to 0.3 μm), more preferably 100nm (0.1 μm to 0.15 μm) to 150 nm (0.12 μm), yet more preferably about120 nm.

When the ultrafine particles of SiO₂ are used, the effect thereof isobserved as could be expected even if the amount of the ultrafineparticles fixed is small. However, the amount of thereof per unit areais 0.01 to 1 mg/cm², preferably 0.1 to 0.3 mg/cm². These upper and lowerlimits are also determined for the same reasons for the above particlediameter, i.e., in view of the anti-reflection effect (lower limit) andthe resolution (upper limit).

The proportion of each of the electrically conductive component (minorcomponent) and the anti-reflection functional component (majorcomponent) varies to some extent depending upon the manufacturingconditions. The proportion of the electrically conductive component ispreferably at least 10% by weight based on the total weight of theultrafine particles (volume ratio of at least 0.1). When this proportionexceeds 50%, the anti-reflection effect may be degraded, and it istherefore required to adjust this proportion to not more than 50%. Whenan anti-reflection film formed from these ultrafine particles is used inan image display device, the electrically conductive component ispreferably transparent. That is because such a component constitutes noobstacle against a light path.

It is not clear how these components to constitute the ultrafineparticles of the present invention form the particle product or whetherthese components necessarily form a constant form depending upon thekinds and performances of the components. The minor component in aparticle form is sometimes included in the major component, and in thiscase, the average particle diameter of the particle formed of the minorcomponent is 0.01 to 0.05 μm.

Typical examples of the anti-reflection functional component includeSiO₂ (silicon dioxide) and magnesium fluoride (MgF₂). Typical examplesof the electrically conductive component include SnO₂ (tin oxide),indium oxide (In₂ O₃) and Sb₂ O₃ (antimony oxide). These electricallyconductive components may be used in combination. The combination of twocomponents is not limited to the combination of the above twocomponents. The point is that the ultrafine particles satisfy the twofunctional requirements. When the minor component is included in themajor component as described above, such a state is comparable to astate in which the ultrafine particle(s) composed of the major componentis (are) the sea and the ultrafine particles of the minor component arepresent like islands. Further, when not more than 10% by weight of fineparticles which have an average particle diameter of 0.01 to 0.05 μm andare composed of an electrically conductive component or a combination ofan electrically conductive component with an anti-reflection functionalcomponent are incorporated to the ultrafine particles of the presentinvention, there is obtained the same effect as that obtained by usingthe ultrafine particles of the present invention alone.

The ultrafine particles used in the present invention can be usuallyproduced from metal components. As an apparatus for the production ofthe ultrafine particles, there is employed an apparatus in which theanti-reflection functional component and the electrically conductivecomponent are both vaporized by means of the heat source selected fromarc, plasma (induction plasma and arc plasma), laser, electron beam,gas, etc., and the vaporized components are rapidly cooled to produceultrafine particles, each of which contains a mixture of thesecomponents.

In the process for the production of the ultrafine particles by arc,provided by the present invention, there is used an oxygen gas or amixed gas of an oxygen gas and an inert gas (helium gas, argon gas,etc.) as an atmospheric gas in the system, and arc is generated betweenraw materials for the ultrafine particles and a discharging electrodeprovided askew or at right angles with the raw materials to formultrafine particles, each of which is a mixture of oxides of the rawmaterials.

More specifically, there may be used ultrafine particles producingapparatus using a laser, described in U.S. Pat. No. 4,619,691 andultrafine particles generating apparatus using arc, described in U.S.Pat. Nos. 4,610,718 and 4,732,369.

Besides the above apparatus, the ultrafine particles can be naturallyproduced by a chemical method.

The ultrafine particles of the present invention can be produced withoutany problem by operating the above apparatus according to a conventionalmethod. The ultrafine particles of a mixture of oxides can be producedfrom a mixture of at least two materials this case, when materialshaving nearly the same vaporization rate are mixed, there can be formedultrafine particles having a concentration of oxides close to thecomposition ratio of the mixed materials.

The ultrafine particles of oxides are formed not only when the rawmaterials are metals but when the raw materials are also metal oxides.In this case, when the metal materials tend to easily form a component,ultrafine particles of the compound are formed. When the metal materialshardly tend to form a compound, ultrafine particles of one oxide andultrafine particles of the other oxide are formed. Since an oxide havingelectrical conductivity and an oxide having an anti-reflection functionusually do not form a compound, there is formed a mixture of ultrafineparticles of one oxide and ultrafine particles of the other oxide.

Examples of the ultrafine particles having an infrared reflectionfunction, an electromagnetic wave shielding function or a clearelectrically conductive function include metal oxides such as TiO₂,ZrO₂, SnO₂ and In₂ O₃ and mixtures of these. Preferred is SnO₂ +10 wt. %Sb₂ O₃ or In₂ O₃ +5 wt. % SnO₂, since this combination is excellent inelectrically conductive properties and infrared reflectioncharacteristics. The film thickness is preferably 0.2 to 0.5 μm, and theparticle diameter is preferably 0.01 to 0.05 μm.

[Base plate, Substrate)

The material of the base plate or substrate is any one of glass,plastics, metals and ceramics, and may be in any form of a plate, afilm, a cube and others. The present invention can be applied to planeand curved surfaces. Examples of the main component of the plastic plateinclude polyethylene, polypropylene, urethane, acryl, phenol, epoxy,melamine, nylon, polyimide, polycarbonate, butyl, epoxyphenol, vinylchloride and polyester. The surface of the base plate on which a film ofthe ultrafine particles may not only be planar, but also has a curvaturelike a Braun tube. Further, a film of the ultrafine particles may beformed on one surface or both surfaces.

(Film of ultrafine particles)

The film of the present invention is composed mainly of the aboveultrafine particles. In addition, when the material components of theabove ultrafine particles are finer ultrafine particles (averageparticle diameter of 0.01 to 0.05 μm), a mixture of the ultrafineparticles of the present invention with the finer ultrafine particles isalso included in the scope of the present invention.

As a thin film, it is sufficient to form one layer of the ultrafineparticles, any yet two layers of the ultrafine particles may be formedas required. The thin film preferably has a thickness of 0.1 to 0.2 μm.Whether the thin film is formed of one layer or whether it is formed oftwo layers, the film thickness is preferably not more than 0.3 μm.

The optimum mixing ratio of the electrically conductive component andthe anti-reflection functional component is the same as that describedconcerning the production of the above ultrafine particles. Theformation of a thin film of the mixed ultrafine particles of theelectrically conductive component and the anti-reflection functionalcomponent can be carried out by coating a suitable amount of theultrafine particles on a base plate. In view of operability andeconomical benefit, it is preferred to form one layer. The concavesformed among the ultrafine particles are preferably 0.05 to 0.1 μm deep.The distance between adjacent electrically conductive componentparticles is preferably not more than 0.05 μm.

The thin film is formed by dispersing the ultrafine particles of thepresent invention or a combination of the ultrafine particles withultrafine particles as raw materials in a solution of Si(OR)₄ (in whichR is an alkyl group) in an alcohol, applying the resultant solution ontoa translucent image display plate, heating (firing) the applied surfaceto form a film of the ultrafine particles coated with an SiO₂ formed byhydrolysis of the Si(OR)₄. The SiO₂ which is a hydrolysis product fromSi(OR)₄ also works as a binder since it penetrates gaps between the baseplate and each ultrafine particle.

The "R" in the above Si(OR)₄ is preferably an alkyl group having 1 to 8carbon atoms, particularly preferably an alkyl group having 1 to 5carbon atoms. The Si(OR)₄ alcohol solution shows an increase inviscosity with an increase in the number of carbon atoms. Therefore, thealcohol in which the Si(OR)₄ is to be dissolved is suitably selected soas not to add to the viscosity in view of operability. The alcohol isgenerally selected from alcohols having 1 to 5 carbon atoms.

A salt of a metal belonging to the groups II and III of the periodictable may be incorporated to impart the above thin film with anantistatic effect. Typical examples of the salt include hydrochloride,nitrate and carboxylate of aluminum.

Further, when the solution for forming a thin film is prepared, water asa promoter for the hydrolysis of Si(OR)₄ and a mineral acid such asnitric acid as a catalyst may be incorporated.

The above-prepared alcohol solution may be coated by any one of aspinning method, a dipping method, a spray method and a combination ofthese. The heat treatment of the coated surface may be practicallycarried out at a temperature in the range of 50° to 200° C.

It is practical to form a clear electrically conductive film as ananti-reflection film. In this case, the clear electrically conductivefilm constitutes an undercoat for the anti-reflection film. In practicalpoint of view, the thickness of the anti-reflection is preferably notmore than 200 nm (0.2 μm), more preferably 5 to 50 nm (0.005 to 0.05μm), although the thickness depends on raw materials for the film. Theclear electrically conductive film is constituted of an electricallyconductive metal oxide film formed from at least one of SnO₂, In₂ O₃ andSb₂ O₃. Beside this, the clear electrically conductive film may be anelectrical conductivity-imparted film produced by incorporating at leastone of the above clear electrically conductive metal oxides and metalsalts having hygroscopicity into an SiO₂ thin film.

The above metal salt having hygroscopicity, to be contained in the SiO₂thin film, may be selected from inorganic acids such as hydrochlorides,nitrates, sulfates and organic acids such as carboxylates. Preferred aresalts of metal elements belonging to the group II of the periodic tabletypified by magnesium and salts of metal elements belonging to the groupIII of the periodic table typified by aluminum. These metal salts absorbwater in the atmosphere to reduce the electric resistance on the panelsurface.

On the other hand, the electrically conductive metal oxide per se haselectrical conductivity and it is therefore more preferred than themetal salts to reduce the electric resistance on the panel surface. Whenthe contents of the above metal oxide and the metal salt in the SiO₂ areeven small, their effects are observed to a certain effect. However,this content per unit area of the SiO₂ thin film is preferably 0.01 to1.0 mg/cm², more preferably 0.15 to 0.3 mg/cm². The lower limit of theabove content is determined in view of an effect on a decrease in theelectrical conductivity, and the upper limit thereof is determined inview of strength of adhesion to the panel surface.

The electrically conductive undercoat film is required to have thin filmproperties which hardly influence the performance of the anti-reflectionfilm formed thereon. The above film of the present invention satisfythis condition.

The step of forming the above clear electrically conductive film will bedetailed below. Since the clear electrically conductive film is formedon the panel of a cathode ray tube (image display face plate), it isdesirable to form it at such a temperature that does not exert a stresson a glass plate constituting the panel (not more than about 500° C.).Any method that satisfies this may be employed. Typical examples of themethod of forming the clear electrically conductive film are describedbelow.

i) The method of directly forming a film of the electrically conductivemetal oxide of at least one of SnO₂, InO₂ O₃ and Sb₂ O₃ on the glasspanel includes (1) a method in which the metal oxide is used as a targetand placed so as to face the panel surface in an sputtering apparatus,and the metal oxide film is formed on the panel surface by sputteringand (2) a method in which the metal oxide film is formed on the panelsurface from an organic metal compound as a raw material by a known CVDmethod. In the above method (2), the organic metal compound includesalkyl metal compounds having the formula of M(R)_(m) or alkoxy metalcompounds having the formula of M(OR)_(m) in which M is tin, indium orantimony, m is the valence of M, and R is an alkyl group (in whichR=C_(n) H_(2n+1) and practically n=1 to 5). Specific examples areSn(CH₃)₄ and Sn(OC₂ H₅)₄.

ii) The method of forming the clear electrically conductive film byincorporating an electrically conductive substance into the SiO₂ filmwill be explained below.

The SiO₂ film can be easily obtained by hydrolyzing alkoxysilane,Si(OR)₄, in which R is an alkyl group having 1 to 5 carbon atoms inpractical point of view. In the present invention, at least one additiveselected from clear electrically conductive metal oxides and metal saltshaving hygroscopicity, detailed in the above invention of the cathoderay tube to achieve the above first object, is added to an alcoholsolution of Si(OR₄), the resultant solution is applied on a panelsurface, the panel surface is heated to hydrolyze the Si(OR₄), wherebyan SiO₂ film is formed. The amount of the above additive is preferably0.05 to 7% by weight based on the alcohol solution, more preferably 1.0to 2.0% by weight.

The above clear electrically conductive oxide is not dissolved in theabove alcohol solution, but is merely dispersed therein, while the metaloxide is partialy or wholly dissolved. The better the above additive isdispersed or dissolved in the above alcohol, the better the electricalconductivity of the resultant SiO₂ film is. In this point of view, it ispreferred to incorporate a ketone such as acetyl acetone or ethylcellosolve as a dispersant to the above solution. To ease the hydrolysisof Si(OR)₄, it is further preferred to add water and an inorganic acidsuch as nitric acid as a catalyst.

The alcohol solvent in which the above Si(OR)₄ is to be dissolved ispreferably an alcohol which is derived from the alkyl, R. The mostpractical combination is where n=2 or R is ethyl, i.e., a combination oftetraethoxysilane (OC₂ H₅)₄ and ethyl alcohol as a solvent.

The above alcohol is applied to the panel by a spinning method, adipping method, a spray method or a combination of these.

The temperature for heat-treatment of the above applied surface for thehydrolysis of Si(OH)₄ to form a SiO₂ thin film is preferably in therange of 50° to 200° C., more preferably 160° to 180° C. Being carriedout at a relatively low temperature as described above, the method offorming this electrically conductive SiO₂ thin film is more advantageousthan the forming method in the above i). When this method is applied toa cathode ray tube such as a Braun tube, a Braun tube can be treated inthe state of a completed tube, and therefore this method is suitable fora mass production process. This method may be naturally applied to theproduction process of a Braun tube before the completion of the tube.

In the method of obtaining the anti-reflection film by forming an unevensurface, the concavoconvex size is preferably about 0.1 μm, and thevolume preferably changes continuously in the depth direction. Due tothis, the refractive index continuously changes, and the anti-reflectioneffect can be obtained. In this case, when uniform SiO₂ fine particleshaving no particle size distribution are used, the fine particles adhereregularly, and no film of which the concavoconvex volume continuouslychanges in the depth direction is obtained. Therefore, littleanti-reflection effect is obtained.

In contrast, when ultrafine particles having a particle sizedistribution are used, the resultant film has proper pores, and as aresult, the concavoconvex volume continuously increases in the depthdirection. There is therefore obtained an anti-reflection effect.Further, when an Si(OR)₄ alcohol solution is used, substances other thanSi in the Si(OR)₄ alcohol solution sublimate at about 150° C., and Siprecipitates to form a film, which has an effect of bonding the glassand the SiO₂ ultrafine particles to each other firmly. On the otherhand, acetyl acetone, acetone and ethyl alcohol which may be mixed withthe Si(OR)₄ alcohol solution have an effect of diluting the Si(OR)₄alcohol solution to control the film thickness of an Si precipitate.

In the present invention, a general chemical method of producingultrafine particles may be applied to the production of the ultrafineparticles of the present invention. In this case, the resultantparticles are uniform, and it is therefore effective to employ aphysical method such as an arc method to obtain ultrafine particleshaving a particle size distribution. In addition, a mixture system ofelectrically conductive particles (InO₂, SnO₂, etc.) and anti-reflectionfunctional particles (SiO₂) is effective. However, when particles eachof which has both properties (e.g., Si--In--O particles) are obtainedinstead of properties of different particles, such particles are notonly free from a decrease in the electrical conductivity but alsoeffectively achieve the anti-reflection effect.

The step of forming an anti-reflection film on the above clearelectrically conductive film as an undercoat will be detailed below.

At first, alkoxysilane, Si(OR)₄, is dissolved in an alcohol. All of theSi(OR)₄ as a raw material and alcohols as a solvent are the same asthose described concerning ii) the formation of the SiO₂ film as anundercoat of the clear electrically conductive film, and the explanationthereof is omitted.

SiO₂ fine particles having a particle diameter of 50 nm (0.05 μm) to 1μm are dispersed in a solution of Si(OR)₄ in an alcohol, prepared in thesame manner as in ii). In view of the anti-reflection effect andresolution, the amount of the above fine particles is preferably 0.1 to10% by weight, more preferably 1 to 3% by weight. To improve thedispersibility of the SiO₂ fine particles and the hydrolyzability ofSi(OH)₄, it is preferred to add a ketone such as acetyl acetone or ethylcellosolve as a dispersant, and it is preferred to add water and aninorganic acid such as nitric acid as a catalyst.

The above Si(OR)₄ is hydrolyzed to form an SiO₂ thin film and work tothe SiO₂ fine particles to the panel surface. When the above alkylgroup, R, has the general formula of C_(n) H_(2n+1), n is practically 1to 5, preferably 2, or R is preferably ethyl. The alcohol in which theabove Si(OR)₄ is to be solved is preferably an alcohol derived from thealkyl, R. The most practical combination is where n=2 or R is ethyl,i.e., a combination of tetraethoxysilane (OC₂ H₅)₄ and ethyl alcohol asa solvent.

When the above Si(OR)₄ alcohol solution containing the SiO₂ dispersionis applied to the undercoat clear electrically conductive film formed onthe panel, there is employed a spinning method, a dipping method, aspray method or a combination of these as is described concerning theformation of the electrically conductive SiO₂ thin film in the aboveii).

The temperature for heat treatment of the above applied surface for thehydrolysis of Si(OR)₄ to form an SiO₂ thin film is preferably in therange of 50° to 200° C., more preferably 160° to 180° C.

The thin film as an anti-reflection material is formed by the abovemethod. Since the above temperature for the above heat treatment iscomparatively as low as that described concerning the method of theformation of the undercoat in ii), this method is suitable for formingthe anti-reflection thin film on the panel surface of a completedcathode ray tube.

The anti-reflection film has fine uneven surface (submicron order) asdescribed above, and such a film does not need the following treatment.However, when the anti-reflection film is formed of uniform ultrafineparticles by a chemical production method, it is difficult to providethe film with such a fine uneven surface. The present inventor hastherefore added an etching treatment to obtain a fine uneven surface.

In this case, when a binder having a higher etching rate than theultrafine particles is used, the binder is gradually removed from thesurface by etching in an etching liquid more actively than are theultrafine particles. Thus, there is obtained an ultrafine particle filmhaving an exact uneven surface in the submicron order. Depending uponetching conditions, the etching liquid is selected from a sodiumhydroxide aqueous solution and a hydrogen fluoride aqueous solution.Since, however, hydrogen fluoride easily removes the ultrafine particlesof SiO₂, etc., as well for a short time, and makes it difficult tocontrol the step, a sodium hydroxide (e.g., 5% aqueous solution) ispreferred. Even if a binder-fired decomposition product obtained afterthe etching with a sodium hydroxide aqueous solution contains SiO₂, thebinder is the more actively removed in the etching liquid than the SiO₂ultrafine particles.

Light reflection occurs in an interface where the refractive indexchanges sharply, and if the refractive index gradually changes in aninterface, reflection no longer occurs. To prevent reflection from sodaglass (refractive index, about 1.53), magnesium fluoride (MgF₂) having alowest refractive index (about 1.38) is deposited thereon by sputtering,etc. However, since the refractive index sharply changes in an interfacebetween a glass base plate and an MgF₂ film and in an interface betweenthe MgF₂ film and air (refractive index, about 1.0), the anti-reflectioneffect is not sufficient. Therefore, a film of which the refractiveindex gradually changes from the value of a glass base plate to thevalue close to that of air efficiently gives an anti-reflection effect.

For the above reason, ultrafine particles of a substance having arefractive index between that of a glass base plate and that of MgF₂,e.g., SiO₂ (refractive index, 1.46) and ultrafine particles of MgF₂ aremixed, and the mixture is applied to a glass base plate. Further, themixing ratio is changed in the film thickness direction. That is, theproportion of the SiO₂ is gradually decreased, and the proportion of theMgF₂ is gradually increased, from the glass base plate surface to thefilm surface, whereby the change in the refractive index in an interfacebetween the coated surface and the glass base plate is moderatelychanged, and an efficient anti-reflection effect is achieved. Accordingto this method, an anti-reflection film having a large area can beformed at a low cost.

The ultrafine particles of a substance having a refractive index closeto that of a glass base plate (e.g., SiO₂) and the ultrafine particlesof a substance having a refractive index close to that of air (e.g.,MgF₂) are mixed, whereby these substances are mixed uniformly at a levelsmaller than the wavelength of light. Therefore, the mixture has anaverage refractive index corresponding to the volume ratio of SiO₂ andMgF₂. That is, in an ultrafine particle film obtained from the mixtureof SiO₂ ultrafine particles and MgF₂ ultrafine particles, the averagerefractive index, n(X), in a position in the film thickness direction isrepresented by n(X)=1.46×V(s) +1.38×{1-V(s)} wherein V(s) is a volumepercentage of the SiO₂ ultrafine particles in the X position. Therefore,when the mixing ratio is changed in the film thickness direction, therefractive index accordingly changes, and the change in the refractiveindex in the interface between the glass base plate and the film changesmoderately.

Further, films having changed mixing ratios are stacked, whereby thereis obtained a film of which the average refractive index graduallychanges as a whole.

As a solution, preferably used is a mixed solution containing an alcoholsolution of Si(OR)₄ (in which R is alkyl having 8 or less carbon atoms,e.g., C₂ H₅ --) and at least one of acetyl acetone, acetone and ethylalcohol.

In the method of forming an anti-reflection film on a glass surface, itis preferred to coat a solvent containing ultrafine particles having aparticle size distribution of which the average particle diameter is notmore than 0.1 μm, fire the coating, and then form an overcoat from amixed solution containing an alcohol solution of Si(OR)₄ and at leastone of acetyl acetone, acetone and ethyl alcohol. The anti-reflectionfilm obtained as above is particularly suitable for an image displaytube.

(Additive)

The additive is added, for example, for an antistatic purpose. Particlesof a hydroscopic metal salt are used as such. Preferred is a salt of atleast one metal element selected from metal elements belonging to thegroups II and III of the periodic table. Practically usable arehydrochlorides, nitrates, sulfates and carboxylates, and at least one isselected therefrom. Particularly preferred are the above salts of atleast one of magnesium and aluminum.

The above metal salts absorb water in the atmosphere to decrease theelectric resistance on the base plate surface. Meanwhile, particles ofelectrically conductive metal oxide are more preferred than the abovemetal salts, since such particles per se have electrical conductivityand decrease the resistance on the base plate surface. As an oxide forsuch particles, practically used is at least one oxide selected fromtin, indium and antimony. That is because these oxides constitute clearelectrically conductive films. In addition, other known electricallyconductive metal oxides, e.g., those having a perovskite structure maybe naturally used. When the content of the above additive fixed is evensmall, its effect is observed to a certain effect. However, this contactper unit area of the base plate is preferably 0.01 to 1.0 mg/cm², morepreferably 0.15 to 0.3 mg/cm². That is, the lower limit of the abovecontent is restricted in view of an effect on a decrease in theelectrical conductivity on the base plate surface, and the upper limitthereof is restricted in view of strength of adhesion to the base platesurface. That is, with an increase in the fixed content, the resistancevalue decreases, but the strength of adhesion decreases to the contrary.

(Pretreatment)

If the wettability to a base plate is considered, it is preferred tocarry out a preliminary treatment with an alkali or fluorine.

(Application method)

The rising or lowering rate of the coating liquid is preferably not morethan 10 mm/s. The base plate may be rested in a container, or the baseplate surface is exposed through a hole provided on the side of acontainer. The latter is suitable for forming a ultrafine particle filmon nearly completed product such as a Braun tube.

The coating liquid-applied surface is practically fired in a furnace ata temperature in the range of 50° to 200° C. It may be also fired byultraviolet light from a high-pressure mercury lamp for a short time.

The above embodiment has been described by way of a dipping method. Whenthe method of application on a plastic base plate or the uniformity ofthe film surface is not a critical matter, it is effective to employanother dipping method, a spinning method, a spray method or acombination of these instead of the above dipping method.

Further, it is effective to apply a coating solution composed mainly ofethyl silicate on the film of the ultrafine particles.

The film may be formed of one layer or at least two layers as required.

(Coating solution)

When the ultrafine particle film of the present invention is formed,there is used a coating solution which contains the predeterminedultrafine particles and the binder, and further contains a couplingagent and other additive as required.

When the translucent plate is a glass product, it is preferred to useSi(OR)₄ (in which R is alkyl) as a binder. When the translucent plate isa plastic, it is preferred to use Si(OR)_(x) (in which x is 2 to 4,particularly preferably 3) as a binder. Further, when the translucentplate is a plastic material, it is preferred to use a coupling agenthaving a functional group which suits the plastic material.

When the translucent plate is a glass product, the ultrafine particlesare dispersed in a solution of Si(OR)₄ (in which R is alkyl) in analcohol. When the translucent plate is a plastic, the ultrafineparticles are dispersed in a solution of a silane coupling agent havinga functional group easily reactive to this polymer of the plastic andSi(OR)_(x) (in which x is 2 to 4, particularly preferably 3) in analcohol or in a solution of a mixture of the above Si(OR)₄ and a silanecoupling agent in an alcohol.

The above solution is applied to a translucent plate and the appliedsurface is heated (or fired) to form a film. Due to this heat treatment,the above Si(OR)₄ or the silane coupling agent is decomposed to SiO₂,etc., to work as an adhesive between the ultrafine particles and thebase plate.

The "R" in the Si(OR)₄ is preferably an alkyl group having 1 to 5 carbonatoms. Meanwhile, the silane-coupling agent is required to be suitablyselected depending upon polymer materials of the translucent plate.

For example, when the translucent plate is formed mainly frompolyethylene, polypropylene, urethane or acryl, a silane coupling agentsuch as vinyltriethoxysilane or γ-methacryloxypropyltrimethoxysilane iseffective. When the plate is formed from phenol, epoxy, melamine,polyamide, polyimide or polycarbonate, a silane coupling agent such asγ-aminopropyltrimethoxysilane is effective. Further, when the plate isformed from butyl, epoxy phenol, vinyl chloride or polyester, a silanecoupling agent such as β,3,4-epoxycyclohexylethyltrimethoxysilane orγ-glycidoxypropyltrimethoxysilane is effective.

The alcohol in which the Si(OR)₄ or the silane coupling agent is to bedissolved shows an increase in viscosity with an increase in the numberof carbon atoms of the above R. In view of operability, therefore, thealcohol can be suitably selected so as not to increase the viscosity toexcess. The generally usable alcohol is selected from alcohols having 1to 5 carbon atoms.

Further, to impart the above film with an antistatic effect, a salt of ametal belonging to the groups II and III of the periodic table may beadded to the coating solution.

Furthermore, to decompose the Si(OR)₄, water and a mineral acid such asnitric acid may be added when the coating solution is prepared.

(Anti-reflection film)

The anti-reflection film of the present invention is characterized inthat a dispersion of ultrafine particles having an anti-reflectionfunction is applied to a substrate to form a film having an unevensurface or a film of which the surface portion is at least porous, orthat aggregates of ultrafine particles form nearly spherical fineparticles of which the gaps form an uneven surface of fine particles.

In this case, it is preferred to form a film having a dispersion ofultrafine particles having an antistatic function in combination withthe ultrafine particles having an anti-reflection function.

In the formation of the uneven surface, each of the concave portions ofthe ultrafine particles preferably has a diameter of not more than 0.05μm, the concavoconvex portions are preferably present all over thesurface formed of the ultrafine particles, and the porosity ispreferably about 50%. In this case, the diameter of each of theultrafine particles is preferably not more than a submicron unit(particularly, not more than 0.1 μm). These conditions also apply whenthe film surface is porous. When, however, aggregates each of which isformed of some ultrafine particles are formed, two methods are availablefor forming fine particles, and the resultant aggregates formed by anyone of these two methods preferably have an average particle diameter ofnot more than 0.2 μm. In one method, "smaller" ultrafine particles areattached around each of relatively "larger" ultrafine particles (e.g.,an average particle diameter of not more than 0.15 μm). In the othermethod, ultrafine particles having nearly the same particle diameter areaggregated.

In the anti-reflection film of the present invention, an ultrafineparticle film is formed on a clear base plate from the ultrafineparticles and a binder which is filled in gaps among the ultrafineparticles, and a coupling agent having a functional group which suits abase plate material is allowed to present among the ultrafine particlesand/or in an interface between the ultrafine particle film and the clearbase plate. Further, it is preferred to apply a coating solutioncomposed mainly of silicic acid onto the ultrafine particle film.Alternatively, without regard to the use of any coupling agent, it maybe employed a method in which an ultrafine particle film is formed onthe clear base plate from the ultrafine particles and a binder which isfilled among the ultrafine particles, and a layer composed mainly ofethyl silicate is formed on the ultrafine particle film. As to the otherprocedures, the techniques on the clear base plate can be directlyapplied.

(Binder)

When the base plate is glass, Si(OR)₄ is used as a binder. When the baseplate is a plastic, R'Si(OR)₃ wherein R' is a functional group usablefor coupling and R is an alkyl group is used as a binder; saidfunctional group should suit a plastic material. That is, when theplastic material is an acrylic resin, for example,γ-methacryloxypropyltrimethoxysilane can be used as a coupling agent.When it is an epoxy resin, γ-glycidoxypropyltrimethoxysilane is used.

(Application)

The technique for forming the anti-reflection film of the presentinvention shall not be limited to clear plates, image display plates,Braun tubes, etc., and can be naturally applied to opaque products,metal products, light-absorbing products, etc., unless these are outsidethe tenor of the present invention.

Examples of the apparatus and devices to which the present invention canbe applied include (1) an image display plate produced by forming a filmof the above ultrafine particles on the surface of a substrate,particularly a clear base plate, or applying the above clear plate oranti-reflection product to the surface of a substrate, particularly aclear base plate, (2) an image display protection plate having the samestructure as that described in (1), (3) a Braun tube provided with aplate-like product described in (1) or (2), other cathode ray tubes,liquid crystal display devices, protection products such as windowpanels, and lenses.

The process for producing a Braun tube is as follows. For example, theclear base plate surface is exposed through an opening portion of acontainer, a mixed solution containing the ultrafine particles and thebinder is introduced into the above container, and the solution level iselevated up or down on the base plate surface in the container at aconstant rate to form an ultrafine particle film on the base platesurface.

The addition, the ultrafine particle film may be formed on each surfaceof the substrate or one surface thereof alone.

Embodiments in which the present invention has been applied to the frontpanel surface (glass face plate) of a Braun tube will be describedhereinafter.

A first embodiment of the device of the present invention is shown inFIG. 1, in which numeral 11 indicates a Braun tube, numeral 12 indicatesa coating solution bath, numeral 14 indicates a pressure-adjustingvalve, numeral 15 indicates an overflowing valve, numeral 16 indicates asolution tank, numeral 17 indicates a solution feed pressure valve, andnumeral 18 indicates a leaking valve.

In the above constitution, the coating solution bath 12 was attached tothe Braun tube 11. In this embodiment, the attaching surface of thecoating solution bath 12 was provided with a packing or an O-ring toprevent the leaking of the coating solution and a pressure gas duringthe application step. Further, in view of operability, the coatingsolution bath was also constituted so as to be sealed by merelyinserting a Braun tube.

Then, the coating solution containing a mixture of ultrafine particleswas introduced into a space formed between the coating solution bath 12and the Braun tube face. For this introduction, the overflowing valve 15and the solution feed pressure valve 17 were opened.

By the above procedure, the coating solution filled in the solution tank16 was pressurized to fill it on the Braun tube face, and part of it wasreflowed into an auxiliary tank through the overflowing valve 15,whereby contaminants, etc., adhering to the Braun tube face or a flowpath were discharged together with the overflowed solution.

Thereafter, the overflowing valve 15 and the solution feed pressurevalve 17 were closed, and the pressure-adjusting valve 14 and theleaking valve 18 were opened, whereby the coating solution 13 filled onthe Braun tube face was charged back into the solution tank 16. In thiscase, due to the gas pressure applied to the pressure-adjusting valve 14and the degree of open/close switching of the leaking valve 18, thelowering rate of the coating solution 13 on the Braun tube face wasadjusted.

The mixing method for the preparation of the above coating solution willbe described below.

Ethyl silicate [Si(OC₂ H₅)₄ ] was dissolved in ethanol, and further, H₂O for hydrolysis and HNO₃ as a catalyst were added to form a solution.Nearly spherical SiO₂ ultrafine particles having a diameter of 120 nm(0.12 μm) in an amount of 10% by weight and 2% by weight of SnO₂ultrafine particles having a particle diameter of 6 nm were added to theabove solution. The pH of the solution was adjusted so that the fineparticles were fully dispersed.

Then, the above solution was filled on the Braun tube by the abovemethod, and the solution level was lowered at a rate of 1.0 mm/s.Thereafter, the resultant coating was fired in air at 150° C. for 30minutes to decompose the [Si(OC₂ H₅)₄ ]. The SiO₂ ultrafine particlesadded to the solution were mutually bonded firmly and at the same timefirmly bonded and fixed to the Braun tube face, since SiO₂ formed by thedecomposition worked as a binder. In this method, a uniform andcontinuously uneven surface of the ultrafine particles could be formedon the Braun tube face.

The Braun tube face having the above film formed was measured for areflectance of light at an incident angle of 5° to give a lowreflectance of 0.08% at a wavelength of 550 nm (0.55 μm) as shown inFIG. 2. In this case, when the Braun tube face was inspected from theside askew to the incident light, a phenomenon of opacification, i.e.,Tyndall phenomenon, was not observed.

On the other hand, the above film was measured for a surface resistancevalue to show 16⁶ Ω/cm², and the antistatic characteristic of the filmwas as shown in FIG. 3, which shows that almost no charge appeared ascompared with a conventional characteristic as a reference.

Then, the antistatic low-reflection film of the present invention wasrubbed with a rubber eraser (50--50 type, supplied by Lion Corp.) 50times under a load of 1 kg. The reflectance changed by about 0.1%, andthere was therefore no problem on its quality.

Ultrafine particles having a diameter of 300 nm (0.3 μm) in an amount of10% by weight based on the total amount of ultrafine particles wereadded to the above solution, and the solution was applied in the samemanner as above. The resultant film showed no change in theanti-reflection effect and the antistatic effect, while its reflectancechanged by 0.02% after the same test with the same rubber eraser. Thus,this film had higher strength against mechanical abrasion. This wasbecause the "larger" ultrafine particles insularly dispersed in the filmworked as a barrier.

In the above process for forming an antistatic and anti-reflection film,a film can be directly formed on a completed Braun tube, and all theremaining procedures are to mixing SiO₂ ultrafine particles and SnO₂ultrafine particles in an existing Si(OR)₄ alcohol solution, apply itand fire the resultant coating. Thus, the above process obviates the useof harmful chemicals such as hydrofluoric acid, etc., and gives productshaving a constant quality at a low cost.

The above embodiment has used Si(OR)₄ in which R was ethyl. However, asdescribed earlier, Si(RO)₄ wherein R is C_(n) H_(m) (m=2n+1) may be usedin the range of n=1 to 5. With an increase in n, the viscosity of thesolution increases a little. Therefore, an alcohol compatible with theincrease can be selected as a solvent in view of operability.

According to this embodiment, a film having excellent anti-reflectioneffect and antistatic function can be formed on an image display plateat one application step as described above. Further, the face plate ofthe present invention obviates the use of harmful chemicals such ashydrofluoric acid, etc., can be produced by a simple and safe process,and is suitable for mass-production and also excellent in contaminationdurability.

Another embodiment will be described by reference to FIG. 7.

Another embodiment is shown in FIG. 7, in which numeral 51 indicates aplurality of clear base plates which are rested on a tool 52 and encasedin a coating solution bath 12. The clear base plate of the Braun tube ofthe above first embodiment has been a glass plate, while the clear baseplate 51 was a plastic plate.

In this embodiment, the attaching surface of the coating solution bath12 was provided with a packing or an O-ring to prevent the leaking ofthe coating solution and a pressure gas during the application step.Then, the coating solution containing a mixture of ultrafine particleswas introduced into the coating solution bath 12. For this introduction,an overflowing valve 15 and a solution feed pressure valve 17 wereopened. By the above procedure, a coating solution 13 filled in asolution tank 16 was pressurized to fill it in the coating solution bath12, and part of it was reflowed into an auxiliary tank through theoverflowing valve. Then, the overflowing valve 15 and the solution feedpressure valve 17 were closed, and a pressure-adjusting valve 14 and aleaking valve 18 were opened, whereby the coating solution 13 wascharged back to the solution tank 15. In this case, due to the gaspressure applied to the pressure-adjusting valve 14 and the degree ofopen/close switching of the leaking valve 18, the lowering rate of thecoating solution 13 on a plurality of the clear base plates wasadjusted.

The mixing method for the preparation of the above coating solution willbe described below.

Ethyl silicate [Si(OC₂ H₅)₄ ] containingγ-methacryloxypropyltrimethoxysilane was dissolved in ethanol, andfurther, H₂ O for hydrolysis and HNO₃ as a catalyst were added to form asolution. Nearly spherical SiO₂ ultrafine particles having a diameter of120 nm (0.12 μm) in an amount of 10% by weight and 2% by weight of SnO₂ultrafine particles having a particle diameter of 6 nm (0.006 μm) wereadded to the above solution. The pH of the solution was adjusted so thatthe fine particles were fully dispersed.

Then, the above solution was filled in the coating solution bath 12, andthe solution level was lowered at a rate of 1.0 mm/s. Thereafter, theresultant coating was fired in air at 150° C. for 30 minutes todecompose the ethyl silicate. The SiO₂ ultrafine particles added to thesolution were mutually bonded firmly and at the same time firmly bondedand fixed to the Braun tube surface, since SiO₂ formed by thedecomposition worked as a binder. In this method, a uniform andcontinuously uneven surface of the ultrafine particles could be formedon the Braun tube surface.

(Ultrafine particles-applied device)

The device for which the thin film of the present invention exhibits itsbest effects includes an image display face formed on a translucent baseplate such as the above thin film glass base plate or an anti-reflectionfilm, and a cathode ray tube into which this image display face platehas been integrated.

The amount of the fixed ultrafine particles of the present invention(particularly when SiO₂ is used as an anti-reflection functionalcomponent) is preferably 0.01 to 1 mg/cm² more preferably 0.1 to 0.3mg/cm².

In the ultrafine particles-applied device as above, the electricallyconductive component is preferably transparent. That is because such acomponent constitutes no obstacle against a light path.

(Others)

When a thin film is formed from composite ultrafine particles formedfrom two components, the function of the minor component continues to beactive as a function of the main (major) component. The effect ofremaining finer ultrafine particles (an included component) is exhibiteddue to a tunnel effect, although the distance among the finer ultrafineparticles is large in view of the distance among the ultrafine particlesbut is not as large as the size of each ultrafine particle.

Concerning the function of the component formed from the minor componentand included in an ultrafine particle in the form of a finer ultrafineparticle, there is a distance between one finer ultrafine particle andanother finer ultrafine particle included in an adjacent ultrafineparticle, but the distance is not greater than the ultrafine particle.Therefore, a tunnel effect is exhibited in terms of electricalconductivity. In this case, the major component achieves the function oflow reflection mainly because of its surface roughness necessarilyproduced due to its viscosity. The electrically conductive componentexhibits electrical conductivity due to its tunnel effect. The thin filmformed from composite ultrafine particles shows higher strength than afilm formed of a laminate of functional components since the peelingsite decreases in number. Further, it also shows an improvement inconjunction of two functions since a tunnel effect is utilized ascompared with a mixture of ultrafine particles each of which has adifferent function.

When the main ultrafine particles are formed from a component having ananti-reflection function, a low reflection function is achieved mainlydue to its surface roughness. The electrically conductive componentexhibits electrical conductivity due to a tunnel effect. The filmstrength is improved further than a film of a laminate of functionalcomponents since the peeling site (potential) decreases in number.Further, the film formed above also shows an improvement in conjunctionof two functions since a tunnel effect is utilized as compared with amixture of ultrafine particles each of which has a different function.

In another method, in the case of one layer of an anti-reflectionfunctional film (which means the absence of electrical conductivity andmixing of no electrically conductive particles), an SiO₂ thin formformed by hydrolysis of Si(OR)₄ coats the uniformly dispersed SiO₂ fineparticles and fixed them to a glass (base plate) surface. This film issubjected to an etching treatment as described above. Due to thisuniformly dispersed SiO₂ fine particles, the anti-reflection effect andthe high resolution of a displayed image are maintained. Further, theSiO₂ film contains an additive, i.e., at least one of a hygroscopicmetal salt and an electrically conductive metal oxide. The formerretains hygroscopicity after the heat treatment for hydrolysis ofSi(OR)₄ (this heat treatment improves the film strength), and works todecrease the resistance value on the base plate surface without losingits performance.

The function of the treatment for electrical conductivity is as follows.That is, due to the electrically conductive oxide, there is observed adecrease in a surface resistance value, which is based on the sameprinciple as that of the clear electrically conductive film. The surfaceresistance value being low, the antistatic function is retained. Asdescribed above, the additive used in the present invention exhibits anantistatic effect. In view of decreasing of the surface resistance valueof the base plate, an electrically conductive metal oxide is excellentover a metal salt. Above all, oxides of tin, indium and antimony arepreferred in that the film transparency is excellent and that highresolution of an image can be also maintained. Differing from oxides,some metal salts are fixed in a film in a dissolved state. In this case,the film transparency is excellent, and such metal salts have a functionto maintain high resolution.

An electrically conductive film used as an undercoat exhibits thefollowing functions.

The clear electrically conductive undercoat film tightly adheres to thepanel surface and produces a function and effect of decreasing theelectric resistance on the panel surface. In a film composed of a metaloxide which itself has electrical conductivity or a film which has astructure in which an electrically conductive metal oxide is dispersedin an SiO₂ film, there is observed a decrease in the surface resistancebased on the same principle as that of a so-called clear electricallyconductive film. Due to this, the antistatic function is maintained.

On the other hand, in the SiO₂ thin film containing a metal salt, thefilm is imparted with electrical conductivity since the metal saltabsorbs and retains water. Even after the heat treatment for hydrolysisof Si(OR)₄ (this heat treatment improves the film strength), the metalsalt maintains hygroscopicity and has a function to decrease theresistance value on the panel surface without loosing its performance.

As an additive incorporated into the SiO₂ film, an electricallyconductive metal oxide is excellent over a metal salt in view ofdecreasing the resistance value on the panel surface. Above all, oxidesof tin, indium and antimony are preferred in that the film transparencyis excellent and that high resolution of an image can be alsomaintained. Differing from oxides, some metal salts are fixed in a filmin a dissolved state. In this case, the film transparency is excellent,and such metal salts have a function to maintain high resolution.

In addition, the front panel (image display face plate) of a cathode raytube such as a Braun tube is electrostatically charged for the followingreason. A thin and uniform aluminum film 4 is deposition-formed on aphosphor applied to the inner surface of a Braun tube. When a highvoltage is applied to the aluminum film, a phenomenon of anelectrostatic charge occurs on the front panel of the Braun tube due toelectrostatic induction.

Ultrafine particles (mainly SiO₂ having an anti-reflection function) aredispersed in an alcohol solution of Si(OR)₄ (in which R is alkyl), andafter the solution is applied onto a base plate, the applied surface isheated (fired) to decompose Si(OR)₄ and form a film in which anultrafine particle film is coated with SiO₂. SiO₂ which is adecomposition product from Si(OR)₄ penetrates gaps among the ultrafineparticles and a gap between the ultrafine particles and the base plateand works as a binder.

The thin film obtained by the above method is etched by a dry or wetmethod for a very short time (from rich few seconds to several tensseconds), whereby the layer rich in SiO₂ which is a decompositionproduct on the film surface is etched to form etching grooves among theultrafine particles. In this manner, a fine concavoconvex form at aultrafine particle level is formed on the entire surface of the film toexhibit the anti-reflection function.

The above alcohol solution is applied to the base plate by a spin coatmethod, a dipping method or a spray method, whereby a large area can betreated easily at a low cost. For the etching after the firing, there isemployed a method of immersion in an NaOH aqueous solution, whereby alarge area can be treated easily at a low cost. Since the film is formedfrom the ultrafine particles, a fine uneven surface is formed, and theanti-reflection effect is further improved. The formation of theanti-reflection film by a coating method obviates an expensive vacuumdeposition apparatus, a film having a larger area can be formed, and thecost can be decreased.

Reflection of light occurs in an interface where the refractive indexchanges sharply. Therefore, when the refractive index gradually changes,no reflection occurs. A film imparted with a refractive indexdistribution on the basis of the above principle is the above-describedheterogeneous film.

When a base plate has a concavoconvex form which is smaller than thewavelength of light, each unit of the concavoconvex form is not regardedas an interface, and it is regarded as a plane having an averagerefractive index corresponding to a volume percentage of the base plateand air. That is, the average refractive index, n_(x), in a position, x,in the film thickness direction is represented by n_(x) =n_(z)·V(_(x))+n_(a) (1-v(_(x))) in which v(_(x)) is the volume percentage ofthe base plate, n_(s) is the reflactive index of the base plate andn_(a) is the refractive index of air. Therefore, when volume percentagev(_(x)), is continuously changed by forming a fine uneven surface, therefractive index is also continuously changed, and a heterogeneous filmis formed, whereby the reflection can be prevented.

When the ultrafine particle film is etched, a concavoconvex form havinga size equal to, or smaller than, the ultrafine particles is formed, anda heterogeneous film is formed, which works as an effectiveanti-reflection film. A concavoconvex forming treatment of ultrafineparticles, formation of a porous surface, and aggregation of ultrafineparticles to form fine particles also similarly work.

(Devising of anti-reflection function ultrafine particles per se)

Embodiments of the anti-reflection function ultrafine particles aredescribed by reference to FIG. 8.

FIG. 8(a) shows a porous surface. The diameter of each pore is not morethan 0.05 μm, the porosity is about 50%, and the pores are formed(nearly uniformly) on an entire spherical surface. The entire sphericalform preferably has an average diameter of not more than 0.1 μm. As oneof the methods for forming a porous surface, a core growth method isavailable. In this case, the mixing ratio of analkoxide-water-acid-alcohol starting material is changed, and thehydrolysis and polymerization are nonuniformly carried out, or thereaction proceeds at a higher rate, whereby portions having a high watercontent are formed in each particle. These particles are fired toevaporate water, whereby porous particles are formed in which pores havebeen left.

FIG. 8(b) shows an aggregate formed by attaching "smaller" ultrafineparticles around a relatively "larger" ultrafine particle (having aradius of not more than 0.15 μm in this case), and the aggregate as awhole preferably has an average diameter of not more than 0.2 μm. Inthis case, the aggregates are formed by mixing A and B in the Figure ina suitable mixing ratio and activating the surface of A by mechanicalfriction to allow the surface to adsorb B (technical fusion).

FIG. 8(c) is a variation of the above (b) type. For example, a solutionof SiO₂ ultrafine particles which are alone dispersed is subjected to afurther reaction of the ultrafine particles to give a solutioncontaining a large amount of secondary aggregates. The reaction mixtureis dried, and mechanically pulverized to give secondary aggregateshaving a predetermined size. The aggregates also preferably have anaverage particle diameter of not more than 0.2 μm.

FIGS. 8(d) and 8(e) are perspective views of the (b) and (c) types. FIG.8(d) shows an embodiment of one aggregate of ultrafine particles havinga diameter of not more than 0.05 μm. FIG. 8(e) shows an embodiment ofone aggregate of ultrafine particles having a diameter of not more than0.1 μm. These aggregates themselves also preferably have an averageparticle diameter of not more than 0.2 μm.

(Embodiment of formation of two-layered film)

FIG. 9 is a cross-sectional view of one embodiment in which two-layeredultrafine particle film of the present invention is formed. FIG. 10shows a change in an average refractive index of the above ultrafineparticle film in the film thickness direction. The ultrafine particleshave a form of any one of the embodiments shown in FIG. 8.

At first, ethyl silicate, [Si(OC₂ H₅)₄ ], was dissolved in ethanol, andfurther, water, isopropyl alcohol and acetyl acetone were added to forma solution. Then, SiO₂ ultrafine particles were added to the solutionand fully dispersed by supersonic vibration. The SiO₂ ultrafineparticles were used in an amount of 1 wt. % in the above solution. Afterthe dispersion of the SiO₂ ultrafine particles, citraconic acid wasadded and fully dissolved. The citraconic acid was used in an amount of0.4 wt. % in the above solution. Thereafter, the mixture was furthersubjected to supersonic vibration to fully disperse the SiO₂ ultrafineparticles and fully mix all the components. The resultant solution isreferred to as "solution A".

A solution B, which had been preliminarily prepared by dispersing MgF₂ultrafine particles and ethyl silicate in ethanol, was added to theabove solution A, and these were fully mixed by supersonic vibration.The amount of the MgF₂ ultrafine particles in the solution B was about 1wt. %. The mixing ratio of the solution A and the solution B was variedto change the mixing ratio of the SiO₂ ultrafine particles and the MgF₂ultrafine particles.

A solution having an SiO₂ ultrafine particle/ MgF₂ ultrafine particlevolume ratio of 7/3 prepared by changing the mixing ratio of thesolution A and the solution B was dropped on a glass plate surface, andapplied uniformly with a spinner. Then, the solution-coated glass platewas kept in air to 40° C. for about 10 minutes to dry the film. After itwas dried, a solution having an SiO₂ ultrafine particle/MgF₂ ultrafineparticle volume ratio of 1/1 was dropped, and applied uniformly with aspinner. Thereafter, the glass solution-coated glass plate was fired inair at 160° C. for 45 minutes to pyrolyze the ethyl silicate to formSiO₂. The SiO₂ ultrafine particles and MgF₂ ultrafine particles werefirmly fixed on the glass base plate due to the SiO₂ formed bypyrolysis.

The cross section of the above-formed ultrafine particle film wasmicroscopically observed to show a film formed as shown in FIG. 9, whichhad a total thickness of about 0.2 μm and formed of a layer (firstlayer) having an SiO₂ ultrafine particles 52/MgF₂ ultrafine particles 51volume ratio of 7/3 and having a thickness of about 0.1 μm and a layer(second) layer having the above ratio of 1/1 and having a thickness ofabout 0.1 μm. The SiO₂ ultrafine particles and the MgF₂ ultrafineparticles were uniformly mixed and densely deposited. Numeral 53indicates the glass base plate.

The change in the average refractive index of the above-obtainedultrafine particle film in the film thickness direction was calculatedon the basis of the SiO₂ ultrafine particles/MgF₂ ultrafine particlesvolume ratio, and the result is shown in FIG. 10, in which a indicatesthe refractive index of air, i.e., about 1.0, b indicates the refractiveindex of the first layer, i.e., about 1.42, c indicates the refractiveindex of the second layer, i.e., 1.44, and d indicates the refractiveindex of soda glass, i.e., about 1.53. The refractive index of theentire film gradually changed, which produced an effect of decreasingthe reflectance in the interface between the film and the glass baseplate. Further, the film was formed from the ultrafine particles and afine uneven surface was therefore formed. As a result, the reflection onthe film surface was decreased.

The above glass base plate on which the ultrafine particle film wasformed and an untreated glass base plate were measured for reflectanceof light having a wavelength of 400 to 700 nm at an incident angle of5°, and the results are shown in FIG. 11, in which I indicates thereflection characteristic of the above glass base plate and II indicatesthat of the untreated glass base plate.

The reflectance of the anti-reflection film of the present invention isdecreased to about 1/4 of that of the untreated glass base plate in theentire wavelength region. As to the transmittance shown as an integralvalue in the wavelength region of 400 to 700 nm, the untreated glassbase plate showed a transmittance of 92%, while the glass base plate onwhich the anti-reflection film of the present invention was formedshowed about 80%. Thus, the anti-reflection film of the presentinvention shows low reflection in the entire visible light region andhigh transmittance. Therefore, the anti-reflection film of the presentinvention is suitable as an anti-reflectance for VDT (visual displayterminal).

In the above embodiment, the film was formed of two layers havingdifferent mixing ratios. When the average refractive index is changedmore gradually by forming a film having more layers, the anti-reflectioneffect is more improved.

According to the above embodiment, a film of which the refractive indexcontinuously changes can be formed by repeating a simple applicationmethod. Therefore, the anti-reflection can be formed at a low cost.Further, there is an effect that an anti-reflection film having a largearea can be easily produced.

(Embodiment of ultrafine particles each of which contains a mixture ofcomponents)

FIG. 12 is a cross-sectional view of an anti-reflection film formed on aglass base plate.

In this embodiment, an ultrafine particle film formed of one layer isformed on a glass base plate 48. The ultrafine particle film is composedmainly of ultrafine particles 46, and each ultrafine particle is amixture of an electrically conductive component 46D and ananti-reflection function component 46C. Further, each particle has aporous surface as shown in a partially taken, enlarged view (otherpatterns shown in FIG. 8 are also usable). The electrically conductivecomponent 46D constitutes what is called finer ultrafine particles, andthese finer ultrafine particles may be also present outside eachultrafine particle 46. These ultrafine particles are coated with an SiO₂thin film 46E. However, the present invention shall not be limited tothis embodiment. That is, the ultrafine particles may be exposed withoutbeing coated with an SiO₂ film. Formed in gaps between each ultrafineparticle and the glass base plate 48 is an SiO₂ filled portion 46F.

The SiO₂ thin film 46E and the SiO₂ filled portion are formed of adecomposition product of Si(OR)₄ obtained by firing.

In this embodiment, SiO₂ is used as an electrically conductivecomponent, and SiO₂, as an anti-reflection function component. The SnO₂/SiO₂ volume ratio is from 0.1 (10%) to 0.5 (50%). In this case, theamount ratio of the electrically conductive component in each ultrafineparticle is from 1% by weight to 50% by weight, and this amount ratio iscalculated without the SiO₂ thin film 46 and the SiO₂ filled portion.

Further, the distance between adjacent ultrafine particles is requiredto be a distance in which one electrically conductive componentcontained in one ultrafine particle and the other electricallyconductive component contained in an adjacent ultrafine particle arepositioned so that a tunnel effect is exhibited. Such a distance ispreferably not more than 0.05 μm.

The average particle diameter of the ultrafine particles (thickness ofone layer) is not more than 0.1 μm. Therefore, the thickness of the thinfilm may be 0.1 to 0.2 μm. In this case, the depth of a dent formed inthe thin film is usually 0.05 to 0.2 μm (the height of a dent is 0.05 to0.2 μm when the ultrafine particles are coated with an SiO₂ thin film).

The SiO₂ which is a decomposition product from Si(OR)₄ penetrates thegap between the ultrafine particles and the thin film. Therefore, italso works as a binder.

The reason for high mechanical strength of the anti-reflection film ofthis embodiment is considered as follows. The SiO₂ film which is formedby the following hydrolysis is present and works as a protection film.##STR1##

A fine, regular and uniform uneven surface is formed on a flat platefrom the ultrafine particles of the present invention. Therefore, anexcellent anti-reflection effect is obtained on the entire film surface,and a decrease in the resolution due to an unnecessary uneven surface nolonger occurs.

(Embodiment of formation of thin film)

The following is an embodiment in which the present invention wasapplied to the front panel (glass face plate) of a Braun tube.

Tetraethoxysilane, [Si(OC₂ H₅)₄ ], was dissolved in ethanol, and water(H₂ O) for hydrolysis and nitric acid (HNO₃) as a catalyst were added toprepare an alcohol solution. Ultrafine particles 1 (having a nearlyspherical particle form) prepared in the same manner as in the aboveembodiment were added to the above alcohol solution at a rate of 1. Atthe same time, a suitable amount of acetyl acetone was also added as adispersant.

Before the ultrafine particles 1 were added to the above alcoholsolution, various additives shown in Table 1 were also added inpredetermined amounts.

                                      TABLE 1                                     __________________________________________________________________________                 Embodiment                                                       Item         2     3     4     5     6                                        __________________________________________________________________________    Mixed solution wt. %                                                          Alcohol solution of                                                                        50    50    50    50    50                                       Si(OC.sub.2 H.sub.5).sub.4 in ethanol                                         Additive                                                                      Al(NO.sub.3).9H.sub.2 O                                                                    0.5   0.5   --    --    --                                       (nitrate)                                                                     AlCl.sub.3   --    0.2   --    --    --                                       (hydrochloride)                                                               Al.sub.2 O(CH.sub.3 COO).sub.4 4H.sub.2 O                                                  --    --    0.7   --    --                                       (carboxylate)                                                                 Dispersant Acetyl acetate                                                                  50    50    50    50    50                                       Fine particle                                                                              1.0   1.0   1.0   1.0   1.0                                      Properties                                                                    Reflectance (%) 5° regular                                                          0.5≧                                                                         0.5≧                                                                         0.5≧                                                                         0.5≧                                                                         0.5≧                              reflectance 555 nm                                                            Surface resistance (Ω)                                                               1 × 10.sup.9 ≧                                                         1 × 10.sup.8 ≧                                                         1 × 10.sup.9 ≧                                                         1 × 10.sup.7 ≧                                                         1 × 10.sup.12 ≧             Strength     +0.1≧                                                                        +0.2≧                                                                        +0.1≧                                                                        +0.2≧                                                                        +0.2≧                             (Increment in reflectance                                                     after rubbing with an                                                         eraser 50 times)                                                              __________________________________________________________________________

The above mixed solution was dropped on a glass face plate and furtheruniformly applied with a spinner.

The mixed solution-applied glass face plate was fired in air at 150° C.for about 30 minutes to decompose the tetraethoxysilane [Si(CO₂ H₅)₄ ].The ultrafine particles added to the alcohol solution were firmly fixedby a continuous uniform SiO₂ thin film formed by the decomposition, andan uneven surface was formed on the glass face plate. The structure ofthe anti-reflection film was the same as that shown in FIG. 12. Numerals46E and 46F indicate an SiO₂ portion formed by the decomposition oftetraethoxysilane, and this portion contained an antistatic component asan additive.

The method of application of the solution shall not be limited to theabove spinning method, and it may be a dipping method, a coating method,a spray method and a combination of these. The temperature for firingthe solution-applied glass face plate is preferably in the range of 50°to 200° C.

The above glass face plate on which the anti-reflection film was formedwas measured for a reflectance of light at an incident angle of 5° toshow a reflectance of not more than 0.5% at a wavelength of 500 nm and areflectance of not more than 1% in the wavelength region of 450 to 650nm as shown in Table 1. These values fully satisfy the conditionrequired for use in VDT (visual display terminal).

The anti-reflection film on the glass face plate was rubbed with aneraser (trade name, Lion 50--50, supplied by Lion Jimuki) under a loadof 1 kg to show an increase in reflectance by only about 0.1 to 0.2% asshown in Table 1. Thus, the anti-reflection film had no problem. Forcomparison, a glass face plate on which an uneven surface was formed byconventional etching was tested in the same manner as above to show thatthe reflectance increased by 2% when it was rubbed only once. When itwas rubbed five times, it showed the same reflectance as that of anuntreated glass face plate.

Further, it is considered that a low surface resistance is obtained asshown in Table 1 for a reason that the antistatic components in thesolution effectively work and do not have much influence on theanti-reflection performance and the film strength.

In the process for forming the above anti-reflection film, completedtube can be directly formed, and the anti-reflection film can be formedonly by adding commercially available SiO₂ fine particles to an existingSi(OR)₄ alcohol solution and applying and firing it. The anti-reflectionfilm can be formed safely at a low cost without using any harmfulchemicals such as hydrofluoric acid, etc.

The ultrafine particles are not necessarily spherical, and they may besemi-spherical. When, however, the diameter of the ultrafine particlesis too small, the outerfiner surface of a formed film is too smooth, anda sufficient anti-reflection effect may not be obtained. Therefore, theaverage particle diameter is preferably at least 50 nm (0.05 μm).Meanwhile, when the diameter is too large, the diffusion effect is toohigh, and the resolution decreases. Further, the film strengthdecreases. Therefore, the average diameter is preferably not more than0.1 μm as defined as so-called ultrafine particles.

The method of application of the Si(OR)₄ alcohol solution containing theultrafine particles may be any one of a dipping method, a coatingmethod, a spray method and a combination of these in addition to theabove spinning method. The firing temperature after the application ispreferably in the range of approximately 50° to 200° C.

The above embodiment has used Si(OR)₄ in which R was ethyl. However, asdescribed earlier, Si(OR)₄ wherein R is C_(n) H_(2n+1) may be used inthe range of n=1 to 5. With an increase in n, the viscosity of thesolution increases a little. Therefore, an alcohol compatible with theincrease can be selected as a solvent in view of operability.

Further, the above embodiment has used a salt of aluminum as a typicalexample of metal salts which are additives for imparting an antistaticeffect. However, when a salt of any one of metal elements belonging tothe groups II and III of the periodic table is used, a similar effectcan be obtained. The above embodiment has used SnO₂ as a typical exampleof the electrically conductive metal oxide. However, the electricallyconductive metal oxide may be selected from other known metal oxidessuch as In₂ O₃ and Sb₂ O₃ and composite metal oxides having a perovskitestructure such as LaNiO₃ and La_(1-x) Sr_(x) CoO₃ (all of these have aspecific resistivity of 10⁻¹ Ω·cm at room temperature).

According to this embodiment, there is obtained an image display faceplate on which an anti-reflection film having excellent anti-reflectioneffect, high mechanical strength and high antistatic performance isformed. Moreover, the face plate of the present invention can beproduced by a safe and simple process without any treatment with aharmful chemical such as hydrofluoric acid, is suitable formass-production and excellent in contamination resistance.

(Embodiment of formation of thin film)

0.2 Gram of the above oxide ultrafine particles having ananti-reflection performance were dispersed in 1 g of nitric acid, and 5g of a silicic ester alcohol solution, 5 g of acetyl acetone and 0.1 gof dicarboxylic acid were added. The resultant mixture was stirred anddispersed. The resultant solution was dropped on a glass base plate andspin-coated at 600 rpm for 1 minute. The coated glass base plate wasfired at 160° C. for 30 minutes. The formed film was measured for aregular reflectance of light at 5° in the visible light region of 400 to700 nm to show 0.06%, and it had a surface resistance of 0.5 to 1×10⁷Ω/cm².

SiO₂ ultrafine particles and SnO₂ +Sb₂ O₃ ultrafine particles wereseparately formed, and a mixture of these materials was prepared. A filmwas formed from the mixture in the same manner as above. The film had asurface resistance of several tens GΩ/cm².

As described above, according to this embodiment, as least two kinds ofoxide ultrafine particles are formed in the form of a nearly uniformmixture by the use of an arc heat source. A film having compositefunctions of electrical conductivity and anti-reflection performance canbe formed from this mixture of the oxide ultrafine particles by applyingit only once.

As the heat source for forming the above mixture of oxide ultrafineparticles, induction plasma or arc plasma is used, and a mixture ofoxide ultrafine particles can be similarly obtained by adding the abovemixed powders to the plasma.

(Embodiment of undercoat clear electrically conductivefilm+anti-reflection film)

An electrically conductive undercoat film was formed on the front panel(glass face plate) of a Braun tube as shown in Embodiments 7 to 10 inTable 2.

In Embodiment 7, the electrically conductive film was formed of SnO₂,and the film formation was carried out by a CVD method under thefollowing conditions.

    ______________________________________                                        Apparatus          Atmospheric pressure                                                          CVD system                                                 Organic tin compound                                                                             Sn(CH.sub.3).sub.4                                         as material                                                                   Dopant             Freon gas                                                                     Chlorofluorocarbon                                         Carrier gas        N.sub.2                                                    Base plate temperature                                                                           350° C.                                             (Glass face plate)                                                            ______________________________________                                    

In Embodiment 8, an SiO₂ thin film contained SnO₂ fine powders as clearelectrically conductive fine powders, and the film was formed asfollows.

    ______________________________________                                        (1) Composition of alcohol solution of                                            alkoxysilane, Si(OR).sub.4 :                                                  Ethyl alcohol (C.sub.2 H.sub.5 OH)                                                                  88 cc                                                   Tetraethoxysilane (Si(OC.sub.2 H.sub.5).sub.4)                                                      6 cc                                                    SnO.sub.2 clear electrically                                                                        1.2 g                                                   conductive fine powders                                                       Water (H.sub.2 O)     6 cc                                                (2) Application of solution                                                                             spinner, 500 rpm                                        to glass face plate                                                       (3) Firing of coated film 160° C., 30 minutes                          ______________________________________                                    

In addition, In₂ O₃ and Sb₂ O₃ were also used alone or in combination asclear electrically conductive fine powders in place of the above SnO₂,and films were formed in the same manner to give nearly the sameresults. Therefore, SnO₂ fine powders were described as a typicalexample.

In Embodiment 9, a composite target of In₂ O₃ and SnO₂ (5 wt. %) wasprepared, and a film was formed by depositing a mixture of In₂ O₃ andSnO₂ on a glass face plate by a high-frequency sputtering method.

In Embodiment 10, an SiO₂ thin film contained aluminum nitrate, Al(NO₃)₃·9H₂ O as a hygroscopic metal salt, and the film was formed as follows.

    ______________________________________                                        (1) Composition of alcohol solution of                                            alkoxysilane, Si(OR).sub.4 :                                                  Ethyl alcohol (C.sub.2 H.sub.5 OH)                                                                  88 cc                                                   Tetraethoxysilane (Si(OC.sub.2 H.sub.5).sub.4)                                                      6 cc                                                    Metal salt, Al(NO.sub.3).sub.3.9H.sub.2 O                                                           1.2 g                                                   Water (H.sub.2 O)     6 cc                                                (2) Application of solution                                                                             spinner, 500 rpm                                        to glass face plate                                                       (3) Firing of coated film 160° C., 30 minutes                          ______________________________________                                    

In addition, AlCl₃, Ca(NO₃)₂, Mg(NO₃)₂, ZnCl₂, etc., were also usedalone or in combination as a metal salt in place of the above aluminumnitrate, and films were formed in the same manner to give nearly thesame results. Therefore, aluminum nitrate was described as a typicalexample.

Then, a thin film as an anti-reflection film was formed on each of theabove-obtained electrically conductive undercoat films in the followingmanner.

Tetraethoxysilane [Si(OC₂ H₅)₄ ] was dissolved in ethanol, and water (H₂O) for hydrolysis and nitric acid (HNO₃) as a catalyst were added toprepare an alcohol solution. SiO₂ ultrafine particles prepared so as tohave a particle diameter of 50 nm to 150 nm (ultrafine particles havinga nearly spherical form) in an amount of 1% by weight were added to theabove alcohol solution. At the same time, a suitable amount of acetylacetone as a dispersant was added to fully disperse the ultrafineparticles.

                                      TABLE 2                                     __________________________________________________________________________                Embodiment                                                        Item        7          8      9       10       Comparison                     __________________________________________________________________________    Undercoat electrically                                                        conductive film wt. %                                                         Main component                                                                            SnO.sub.2  SnO.sub.2                                                                            In.sub.2 O.sub.3 + SnO.sub.2                                                          SiO.sub.2 thin film                                                                    Not formed                                                           containing hygro-                                                             scopic component                        Formation method                                                                          CVD of organotin                                                                         SnO.sub.2 fine                                                                       Sputtering                                                                            Hydrolysis of                                       (Chemical deposition)                                                                    particles +    alkoxysilane                                                   Hydrolysis of                                                                 alkoxysilane                                           Film thickness nm                                                                         10         30     10      100                                     Surface resistance (Ω/□)                                                 10.sup.8   10.sup.8                                                                             10.sup.7                                                                              10.sup.10                               Mixed solution for anti-                                                      reflection film wt. %                                                         Alcohol solution of                                                                       50         50     50      50       50                             Si(OC.sub.2 H.sub.5).sub.4 in ethanol                                         Dispersant  50         50     50      50       50                             (acetyl acetone)                                                              SiO.sub.2 fine particle                                                                   1          1      1       1        1                              Properties                                                                    Reflectance (%)                                                                           0.4≧                                                                              0.3≧                                                                          0.4≧                                                                           0.3≧                                                                            0.3≧                    (5° regular reflection)                                                Strength %  +0.1≧                                                                             +0.1≧                                                                         +0.1≧                                                                          +0.5≧                                                                           +0.1≧                   (increment in reflectance                                                     after rubbing with eraser                                                     50 times)                                                                     Attenuation time (sec)                                                                    10≧ 10     15≧                                                                            25≧                                                                             200≧                    from switching off to                                                         1 kV or less                                                                  __________________________________________________________________________

The mixed solution shown in Table 2 was dropped on the electricallyconductive undercoat film, and uniformly applied with a spinner.

Thereafter, the solution-applied undercoat film was fired in air at 150°C. for about 30 minutes to decompose tetraethoxysilane [Si(OC₂ H₅)₄ ].The SiO₂ fine particles added to the alcohol solution was firmly fixedwith a continuous and uniform SiO₂ thin film formed by thedecomposition.

Then, the resultant product was etched by immersing it in a 5 wt. % NaOHaqueous solution for about 15 seconds, washed with water and subjectedto various tests.

The above-obtained anti-reflection film was measured for a reflectance(regular reflection light strength) of light having a wavelength of 550nm at an incident angle of 5°. As shown in Table 2, the regularreflection light strength was not more than 0.4% when that of anAl-deposition film was taken as 100, and the reflectance was not morethan 1% in the visible light region of 450 to 650 nm. In addition, aspectrophotometer, U-3400 supplied by Hitachi Limited, was used (in thisembodiment and embodiments which will follow). This value fullysatisfies the condition required of VDT (visual display terminal).

Then, the surface of the glass face plate on which the electricallyconductive undercoat film and the anti-reflection film were laminatedwas rubbed with a rubber eraser (trade name, Lion 50--50, supplied byLion Jimuki) forcibly (under a load of 1 kgf, the eraser having a crosssection of 18×10 mm) and uniformly 50 times to show a shift in thereflectance by only about 0.1% to 0.2%. Thus, the product had no problemin quality.

In the test with the rubber eraser, the product was measured for a60-degree mirror face gloss (see JIS K5100) before and after the rubbing50 times.

(Embodiment of the particle size distribution)

The following is an embodiment in which the film was imparted with aparticle size distribution. In this embodiment, patterns shown in FIG. 8were also applied.

FIG. 13 shows the particle size distribution of SiO₂ ultrafineparticles, and the average particle diameter was 450 nm. The SiO₂ultrafine particles had a considerably broad particle size distributionand a specific surface area of 70 to 80 m² /g. These ultrafine particleswere dispersed in a solution of 1% by weight of Si(OR)₄ in a mixture ofan alcohol and 50% acetyl acetone, and the resultant coating solutionwas applied to a glass base plate by a spin coat method. Then, thesolution-applied glass base plate was fired at 160° C. for 30 minutes.

The above coating solution had a composition containing 1 to 2% byweight of SiO₂ and a remainder consisting of Si(OC₂ H₅)₄ and 50% acetylacetone, and coated with a spinner at 600 rpm for 30 seconds, and theresultant coating was dried and fired at 160° C. for 30 minutes.

Due to the use of ultrafine particles having the particle sizedistribution described in this Embodiment, the resultant film had properpores. The film was subjected to the above etching treatment and thenmeasured for a reflection characteristic to show 0.06 to 0.3% in thevisible light region (400 to 700 nm). Further, an Si(OR)₄ alcoholsolution and a 50% acetyl acetone solution were applied on this film,and the resultant coating was fired to give a film having atransmittance of at least 90%. This Embodiment has an effect of givingan excellent anti-reflection film by a simple method.

In addition, it is preferred to wash the glass base plate surface andpreliminarily heat the glass base plate up to about 50° C. before theformation of the anti-reflection film.

What is claimed is:
 1. A plate comprising a base plate, a filmcontaining ultrafine particles provided thereon, and a binder, saidultrafine particles comprising substantially. spherical ultrafineparticles, each of which having been treated to have an uneven surfaceso as to provide the plate with an anti-reflection function and highresolution, wherein said substantially spherical ultrafine particleshaving an uneven surface have an average diameter of not more than 100nm and have pores formed substantially uniformly on their surface, saidpores forming said uneven surface.
 2. A plate according to claim 1,wherein said substantially spherical ultrafine particles have a porosityof about 50%.
 3. A plate according to claim 1, wherein a diameter ofeach of said pores is not more than 50 μm.
 4. A plate according to claim1, wherein said base plate is a transparent base plate.
 5. A platecomprising a base plate, a film containing ultrafine particles providedthereon, and a binder, said ultrafine particles comprising substantiallyspherical ultrafine particles, each of which having been treated to havean uneven surface so as to provide the plate with an anti-reflectionfunction and high resolution, wherein each of said substantiallyspherical ultrafine particles having an uneven surface comprises anaggregate of particles made of the same material, each aggregate havingan average particle diameter of not more than 0.2 μm, gaps between theparticles forming said uneven surface.
 6. A plate according to claim 5,wherein said aggregates comprise a plurality of particles having adiameter of not more than 50 μm.
 7. A plate according to claim 5,wherein said aggregates comprise a plurality of particles having adiameter of not more than 100 μm.
 8. A plate according to claim 5,wherein said base plate is a transparent base plate.
 9. A platecomprising a base plate, a film containing ultrafine particles providedthereon, and a binder, said ultrafine particles comprising substantiallyspherical ultrafine particles, each of said substantially sphericalultrafine particles having been treated to have an uneven surface so asto provide the plate with an anti-reflection function and highresolution, wherein each of said substantially spherical ultrafineparticles comprises an aggregate of particles, wherein each aggregatecomprises a first particle having an average particle diameter of 100 to150 nm and a plurality of second, smaller particles adsorbed on asurface thereof, wherein said second, smaller particles are electricallyconductive.
 10. A plate according to claim 9, wherein said secondsmaller particles are made of a material selected from the groupconsisting of SnO₂, Sb₂ O₃ and In₂ O₃.
 11. A plate according to claim10, wherein said second, smaller particles have an average particlediameter not more than 1/10 of the average particle diameter of saidfirst particle.
 12. A plate according to claim 11, wherein said firstparticle is made of at least one material selected from the groupconsisting of SiO₂ and MgF₂.